A 3-year-old girl has a history of recurrent infections, and her neutrophils can phagocytose but not kill Staphylococcus aureus. Which enzyme is most likely deficient in this patient?

In this case, the clinical presentation suggests a problem with the respiratory burst function of neutrophils. Neutrophils are essential components of the immune system, and their ability to kill pathogens is dependent on their capacity to generate reactive oxygen species (ROS) through a process called the oxidative burst. This process requires the enzyme NADPH oxidase, which catalyzes the production of superoxide, a reactive molecule that is crucial for the killing of bacteria after they have been phagocytosed.

The fact that the child can phagocytose Staphylococcus aureus but cannot effectively kill it indicates that the phagocytic process is functioning, but the subsequent killing mechanism is impaired. This aligns with conditions such as Chronic Granulomatous Disease (CGD), which is characterized by a deficiency in NADPH oxidase. In CGD, despite the ability of neutrophils to engulf bacteria, the inability to produce ROS results in recurrent infections, particularly with catalase-positive organisms like Staphylococcus aureus.

The other enzymes listed—catalase, elastase, and superoxide dismutase—do not directly pertain to the oxidative burst mechanism that is critical for the innate immune response and the specific killing of pathogens. Therefore,